Abstract
The development of aluminium alloys capable of elevated temperature service by utilization of Rapid Solidification (RS) processing techniques has been an area of intense study during the past decade. The ternary alloy system, Al-Cr-Fe, has displayed very promising mechanical properties when produced by an evaporation-vapour deposition technique, but little indication exists of whether similar properties may be achievable by using other, more commercially applicable, RS techniques. This work comprises a study of the microstructure and tensile properties of a range of Al-Cr-Fe alloys (Al-2.93 to 8.43 wt% Cr-0.67 to 2.07 wt% Fe) processed by high pressure gas atomisation (HPGA), supplemented by additional experiments using melt-spinning and chill-casting techniques in order to allow the effects of a wide range of cooling rates (<1k/s to 106K/s) to be examined. A series of binary Al-Cr alloys (Al-1.99 to 5.50 wt% Cr) were also processed in order to facilitate comparison with the ternary alloys. The variety of microstructures observed in the atomised powders were consistent with the different cooling rates and nucleation temperatures experienced by droplets of different diameters. The cooling rate necessary to suppress formation of the Ali3Cr2 phase was found to be consistently higher in the ternary alloys than in the corresponding binaries. No evidence was found of partitionless solidification having occurred in the Al-Cr-Fe powders, the estimated solidification front velocities being insufficient to prevent the partitioning of the iron to the infinity-aluminimum cell boundaries. The infinity-aluminium phase was found to be divided into two co-existing, but distinct, solid solutions with different chromium solute levels. A fine spherical phase was observed in the finest particles of the most concentrated alloy (Al-8.43 wt% Cr-1.26 wt % Fe), which was found to be similar to the icosahedral phase identified in the melt-spun ribbon. A relationship has been determined between the thickness of wedge shaped chill castings and powder particle diameters for similar microstructural transitions. A procedure is proposed by which the determination of alloy compositions designed to give a particular microstructure in a specific powder size fraction can be accomplished by a simple chill casting technique. The mechanical properties of the atomised powders consolidated by extrusion have been determined and related to the microstructures and strengthening mechanisms operative in the materials. A large contribution to the strength of the extruded materials is derived from their fine grain size. However, none of the atomised Al-Cr-Fe alloys matched the properties obtained in the Al-7.5 wt% Cr-1.2 wt% Fe alloy material produced by the vapour deposition technique. None of the alloys investigated retained its strength after prolonged thermal exposure at or above 350°C, nor possessed adequate tensile strength properties at 350°C. The results do suggest, however, that a service temperature of 300°C may be possible.